The present invention relates to a sheet feed device for a printer, copier or similar image forming equipment.
It is a common practice with a sheet feed device of image forming equipment to feed a stack of sheets one by one from a rack or table toward a pair of register rollers movable into and out of contact with each other. The register rollers drive the sheet to an image forming section where an ink drum or a photoconductive element, for example, is located. In this type of sheet feed device, a pick-up roller associated with the rack is rotated in such an amount that the sheet moves a distance greater than the distance between the pickup roller and the register roller pair. Hence, after the leading edge of the sheet has abutted against the register roller pair, the leading edge portion of the sheet is sequentially slackened until the pick-up roller stops rotating. This allows the register roller pair to start driving the sheet at an accurate time and corrects the skew of the sheet which may occur during transport, despite the difference between the coefficient of friction of the sheet and that of the pick-up roller and the irregularity in the transport resistance of the path. The sheet feed device may be provided with a drive source implemented by a motor which drives the ink drum or the photoconductive element, a speed changing mechanism in the form of a gear train interlocked with the drive source, a cam for bringing the register roller pair into and out of contact, and a drive transmission mechanism for the cam.
The sheet feed device of the type described has various problems left unsolved, as follows. To begin with, the structure for driving the pick-up roller and register roller needs a great number of parts and is not easy to design due to complexity. Although such parts may be accurately assembled as designed, it is likely that the interlocked relation is disturbed due to their mechanical wear and fatigue as the device is repetitively operated. Then, even when the pick-up roller is rotated a predetermined amount, the actual displacement of the sheet becomes short and prevents the sheet abutting against the register roller pair from being sufficiently slackened. If the abutment of the sheet against the register roller pair which is based on the slack or elasticity of the sheet is not sufficient, the register roller pair is apt to start feeding the sheet at an unexpected time or to fail to correct the skew of the sheet. This brings the sheet out of register with an image provided on the ink drum or the photoconductive element. The misregister of the sheet and image is particularly serious when the print speed or the copy speed is changed.
Another function available with the register roller pair is to change the position on the sheet where an image begins to be transferred in a range corresponding to the upstream side with respect to a sheet feed direction, i.e., in the top-and-bottom direction. With this function, it is possible to change the area of a blank portion or margin to be formed at opposite ends of the sheet with respect to the sheet feed direction. This can be done if the time when the register roller pair starts feeding the sheet is changed. However, the register roller having such a function has to be provided with a number of constituents for transmitting a drive force from the drive source associated with the ink drum or the photoconductive element, as stated earlier. Moreover, the register roller needs a cam for changing the drive timing thereof, a drive source for operating the cam, an encoder or a sensor for determining the current displacement of an image in the top-and-bottom direction, noticeably scaling up and complicating the structure. In addition, since the interengagement of the constituent parts is apt to get out of order, the displacement of an image which can be set is limited to a certain range, resulting in low resolution. Therefore, to change the displacement, it is sometimes necessary to change the design itself.